This invention relates to fixed wing aircraft and, more particularly, to the configuration and method for controlling the flight of aircraft when the primary lifting surface is in deep stall.
It has generally been considered undesirable in normal operation of fixed wing aircraft to operate in deep stall. This condition occurs when the primary lifting surface, the wing, is at so positive an angle relative to the oncoming air flow that flow line separation occurs along essentially the entire upper wing surface and lifting force is lost. The result of such deep stall is generally an uncontrollable and rapid loss of altitude, often accompanied by change in attitude of the aircraft. Control is not regained except as air speed is increased by advancing engine thrust or initiating steep dive into an accelerating drop in altitude to restore normal airflow over lifting and control surfaces. Such flight conditions have ordinarily been avoided, especially at low altitude since many aircraft have insufficient thrust available to regain control and power out of the dive rapidly enough to avoid a crash. Still other aircraft are susceptible to a deep stall condition from which no recovery is attainable by any techniques available in the prior art. In such cases, the aircraft will crash with attendant damage to the craft and high probability of damage or death for occupants.
Typically, the hazard is high under the low speed and low altitude conditions of landing, where high wing lift is necessarily produced by flying close to wing stall conditions. The unintended incidence or inducement of a stall due to air turbulence or over-control might leave little or no time or room for recovery of control--with damaging consequences. The seriousness of the problem is further intensified when landings are required on short runways or over undeveloped landing areas.
Prior art has concerned itself with the role played by the tail surfaces, notably the stabilizer, in efficiently developing the highly positive angle of incidence required of the wing in low-speed flight conditions. The downward force generated at the rear of the fuselage by the stabilizer is used to rotate the aircraft to the wing's maximum unstalled angle of attack. On conventional airplanes having substantially horizontal stabilizers, the downward thrust of the tail section is developed by upwardly tilting the elevator flaps hinged to the rear of the fixed stabilizer surface. However, in this slightly nose-up position of the airplane, the raised elevator flaps cause a downward thrust while, concurrently, the horizontal stabilizer section fixed in relation to the fuselage, generates increased lift (as compared to level flight) in opposition to the raised flaps. To overcome this inefficiency of opposed forces and to increase the down thrust while using smaller tail surfaces, fully tiltable stabilizers have been developed where the entire stabilizer surface rotates leading edge down, relative to the level flight axis of the fuselage. U.S. Pat. Nos. 2,563,757; 2,719,014; and 3,138,353 are illustrative of prior art utilizing tiltable stabilizer surfaces for the double purposes to more efficiently provide downward thrust at the rear of the fuselage and to simultaneously prevent the occurence of stall in an aircraft flying at low speeds.
In none of the prior art is there a method or purpose disclosed to fly an aircraft with the wing fully stalled. Primarily, this is because flight in stalled condition has heretofore been considered uncontrollable and therefore useless or worse. In the past any stall was to be avoided if possible, and remedied rapidly as possible, should such a condition occur inadvertently. The prior art has gone to considerable pains to prevent stalls, and has tended to prevent at almost any cost a deep stall condition with primary lifting surfaces fully stalled.
What is needed is an airplane having essentially normal level flight performance combined with a capability and method of controlled slow descent and landings which are near vertical.
Also needed is an airplane having essentially normal level flight performance combined with a capability and method of recovery from unstable and undesirable flight conditions such as stalls, spins, inverted flight, and the like.